More than thirty years ago, Bell Labs first proposed a device that used focused laser light to either attract or repulse objects. These devices have become known as optical tweezers and are now a key instrument in manipulating matter in both biology and quantum optic applications. But until recently, these devices couldn’t manipulate particles smaller than a few hundred nanometers.

The key technological breakthrough for enabling these optical tweezers to reach deeper into the nanoscale and become so-called “nanotweezers” has been plasmonics. Plasmonics is a field that exploits the surface plasmons that are generated when photons hit a metal structure. Those plasmons move along the surface of the material and create waves or oscillations of electrons.

Now researchers at the Indian Institute of Science (IISc) in Bangalore have added another wrinkle to plasmonics-based nanotweezers by integrating them with magnetic, helical microrobots to overcome some of the limitations these devices have had in trapping objects.

Prior plasmonic-based nanotweezers used forces generated from an enhanced electromagnetic field that occur near plasmonic nanostructures when illumination with a laser of a certain power level hits them, according to Ambarish Ghosh, associate professor at IISc and co-author of the paper in Science Robotics.

One limitation of this arrangement is that these metal nanostructures are fixed on a substrate. And trapping can only occur at certain spots on the substrate's surface, which is a clear drawback, according to Ghosh who worked with Souvik Ghosh, a PhD student at IISc, on the research. “Also one has to wait for the particles to diffuse into such static traps which is a probabilistic event,” added A. Ghosh.

Ghosh claims that by combining plasmonic trapping with magnetically-driven microbots, his team is able to trap, transport, release, and position sub-micron fluid environments in which particles are suspended—known as collodial fluids. “Crucially we can do this with speed and spatial resolution that is simply not possible with any existing technique,” he added.

Gif: Ghosh and Ghose/Science Robotics
This GIF shows silica beads being captured and released by a mobile nanotweezer.

The mobile nanotweezers (MNTs)—as the researchers have dubbed the new devices—consist of a screw-shaped magnetic microrobot that is dotted with silver particles that cluster in response to light. The magnetic microrobots spin about and are directed in their environment through the force of a magnetic field. When different light intensities strike the microrobots, it triggers the silver particles into a plasmonic effect that activates the picking up and then releasing of their targets.

A. Ghosh thinks the most immediate impact will be in the area of bio/microfluidics since the technique allows them to manipulate living sub-micron sized bacteria with minimally invasive optical powers.

In future steps, Ghosh thinks there will be potential to use nanotweezers for nano-assembly applications, such as assembling fluorescent particles in cavities to make nanolasers or sorting and assembling a collection of nanodiamonds for various quantum technologies. “This may require parallelizing the nanotweezers, so that a collection of them can sort and assemble in parallel, just like a bunch of robots would work in a car factory,” he said.